1. Field of the Invention
The present invention relates to a high fidelity regenerative amplifier with a phase correction circuit which can correct a frequency characteristic and a phase characteristic in accordance with a sound field characteristic to obtain regenerated sound extremely near to natural sound specifically in a place where an acoustic condition is not so good, for example, such as in a room of an automobile.
2. Description of the Prior Art
Recently, an amplifier named a so-called audio amplifier capable of regenerating sound with high fidelity is generally used.
The audio amplifier is required to regenerate sound with high fidelity and accordingly a frequency characteristic thereof is adapted to be flat over as a wide frequency range as possible.
A frequency characteristic of a common audio amplifier is substantially flat from about 30 Hz to about 10 KHz while the frequency characteristic is lower with decrease or increase of the frequency in a frequency range other than the above frequency range. On the other hand, there is a phase characteristic that a phase is gradually delayed with increase of the frequency.
In an audio amplifier, heretofore, the quality of regenerated sound has been improved. The improvement of the sound quality is achieved by an equalizer or the like to attain a flat frequency characteristic in a sound field. However, it has been found by a recent experiment that it is desirable that a phase characteristic is flat over the all frequency range in addition to the flat frequency characteristic in order to realize higher quality sound in a general sound field. Heretofore, an improvement in an amplifier for the phase characteristic which is corrected to be flat over the all frequency range is not quite made. Accordingly, a conventional audio amplifier has a limit to improvement of the quality of regenerated sound. Further, since the conventional audio amplifier has a different phase for each frequency, an oscillation may occur in a ultra-low or ultra-high frequency range in the case where the quality of sound is improved on the basis of a conventional idea. Even in this point, there is a limit to improvement of the quality of regenerated sound.
Even in a conventional audio amplifier provided in a vehicle, an equalizer capable of correcting the sound field characteristic in a room of the vehicle is provided in an amplifier so that music can be heard in a relatively good condition even in the room of the vehicle where an acoustic condition is not so good. However, in such a conventional equalizer, since the frequency characteristic can be made flat but a phase characteristic is not taken into consideration, there is also a limit to improvement of the quality of regenerated sound for the above-mentioned reason.
More particularly, an equalizer is generally configured by a combination of different filter circuits having various characteristics. In a conventional equalizer, when a single sound of a sine wave is employed and its frequency is gradually increased or decreased so that a phase characteristic of the equalizer is measured by a measuring apparatus, there can be obtained a phase characteristic having no variation over substantially all frequency range apparently. However, since the phase characteristic is obtained as a result of a combination of phase characteristics of a low pass filter, a band pass filter and a high pass filter constituting the equalizer, a phase distortion occurs due to a phase difference in a frequency range in which phases of each of the filters are overlapped to thereby cause deterioration and impurity of the quality of sound and indefiniteness of orientation of the acoustic image. The phase difference is particularly large at an overlap point of the frequency characteristic curves of the filters, that is, in the vicinity of a dip point. This is caused by the fact that the equalizer includes a feedback circuit.
As described above, the improvement of the sound quality in the prior art resides in the realization of the flat frequency characteristic. However, in order to reproduce the higher quality sound, the phase distortion can not be neglected as described above. The phase of regenerated sound is an important improvement item since it is an important element to the orientation of the acoustic image when excellent sound in the psychological aspect which is not measured by any measuring apparatus, directly speaking, natural sound, such as sound having presence or transparency, or sound which does not give fatigue even if listening to the sound for a long time is pursued.